The family of missing 18-year-old bushwalker Lily Hooper say they are "absolutely devastated" that she hasn't been found after a multi-agency search. This was reported by Qazaqyia.kz citing The Guardian.

On Tuesday, search efforts for the teenager continued through the dense tree canopy and rugged terrain of Nattai national park, about 90km south-west of Sydney, for a seventh day.

"Our family is absolutely devastated that our beloved daughter Lily is missing," the family said in a statement on Tuesday afternoon. "We desperately want to bring Lily home safe and we ask as the search continues, to keep our family and Lily in your thoughts."

Hooper left her Sydney home on Wednesday 12 August for a hike. Initial police inquiries indicate she reached the summit of Bonnum Pic mountain mid-morning on Wednesday, having sent messages from the trail earlier that day.

She was reported missing when she did not return home that evening. Her car was located by emergency services near the trail head on Wanganderry Road at High Range at 11pm on Wednesday.

As of Tuesday afternoon, police said no trace of Hooper or her belongings had been found.

On Monday, police said the search would focus on the lower sections of the Bonnum Pic walking trail.

Nattai national park is about 90km south-west of Sydney.

NSW police chief inspector Brendan Bernie confirmed on Monday that thermal and aerial sweeps by PolAir late on Sunday evening were unsuccessful.

"We've just found no trace of Lily," he said. "It's quite dense with the bushland and there's a dense tree canopy which also hinders our aerial searching."

Police were uncertain of what supplies Hooper had with her, he said.

"We're hopefully relying on Lily's experience to get her through this ordeal and can locate her safely."

Poor weather over the weekend hampered search operations in the already complex terrain.

"We would like to thank all the emergency services and volunteers for their support and continued commitment to the search, not only during the day but throughout the night," the Hooper family said on Tuesday.

They also thanked those who offered to help search, but said due to "terrain and conditions we ask just professional search teams continue".

The search team now includes PolAir, NSW Police Rescue, the SES, RFS, Fire and Rescue NSW, and National Parks and Wildlife Service. Volunteers on the ground include Hooper's friends, family and Volunteer Rescue Association crews mobilised from as far north as Cessnock.

The family asked for privacy and understanding as they navigated the "unimaginable situation".

Hooper is described as Caucasian, approximately 175cm tall, of slim build, with long blond hair and blue eyes. She was last seen wearing a shirt and shorts.

Anyone with information or who walked the Bonnum Pic trail on Wednesday is urged to contact Crime Stoppers on 1800 333 000.